1. Field of the Invention
The present invention relates to a microwave plasma processing apparatus and a method of supplying microwaves, and more particularly, to a microwave plasma processing apparatus that performs plasma-processing on an object by exciting a gas by electric field energy of microwaves emitted from a radial line slot antenna (RLSA), and a method of supplying microwaves using the microwave plasma processing apparatus.
2. Description of the Related Art
Microwave plasma is generated by introducing microwaves into a processing container in a depressurized state and by exciting a gas by electric field energy of the introduced microwaves. In microwave plasma processing apparatuses, when an electron density of plasma is higher than a cut-off density, microwaves cannot be introduced into plasma and thus, are propagated between a dielectric plate and plasma, and some of the microwaves are absorbed into plasma and are used to sustain plasma.
According to the principle of generating plasma, since microwave plasma has a higher electron density Ne and a lower electron temperature Te than plasma that is generated by a capacitively coupled plasma or inductively coupled plasma processing apparatus, a high-quality product can be manufactured at high speed and with less damage by performing plasma processing.
As one of microwave plasma generating apparatuses, a microwave plasma processing apparatus using a radial line slot antenna (RLSA) has been proposed (e.g., see Reference 1). The RLSA is disposed on a dielectric window in a state in which, above a slot plate having a disk shape in which a plurality of slots are formed, a wavelength-shortening plate having the same shape is placed. And the middle part of the RLSA is connected to a coaxial waveguide.
In the above-described structure, microwaves of 2.45 GHz, for example, outputted from a microwave source, are propagated into the coaxial waveguide and are propagated to have a radiation shape in a radial direction of the RLSA. As such, microwaves having a high electric field strength can be radiated into the processing container via the dielectric window from the plurality of slots formed in the slot plate.    [Reference 1] Japanese Laid-Open Patent Publication No. hei 9-63793
However, during processes, the processing container is maintained at a high temperature of more than 200° C. Thus, during the processes, even though a circumferential part of an RLSA 905 is cooled by a cooling jacket 210, the temperature of the RLSA 905 increases to 150° C.-165° C., and the temperature of the cooling jacket 210 placed above the RLSA 905 increases to 80° C.-100° C., and the temperature of an outer conductor 340 increases to 40° C.-60° C., and a temperature of more than 100° C. is maintained even near the outer conductor 340 in some of the processes. As a result, the RLSA 905 and its upper members, such as the cooling jacket 210, the outer conductor 340 of the coaxial waveguide, a rectangular waveguide 305, etc., shown in FIG. 6 are thermally expanded.
Among the members, a wavelength-shortening plate 905a of the RLSA 905 is formed of a dielectric substance such as alumina (Al2O3). Meanwhile, the cooling jacket 210, the outer conductor 340, the rectangular waveguide 305, and a coaxial converter 310, placed above the RLSA 905, are formed of metal such as copper (Cu) or aluminum (Al). Compared to the linear expansion coefficient of alumina which is 7.0×10−6(/° C.), the linear expansion coefficient of copper is 16.7×10−6(/° C.) and the linear expansion coefficient of aluminum is 23.5×10−6(/° C.), which are more than twice that of alumina. Thus, after a temperature increase, each of the cooling jacket 210, the outer conductor 340, and the rectangular waveguide 305, which are placed above the RLSA 905, is expanded and is displaced in an upper direction compared to a state before a temperature increase, as illustrated in FIG. 6.
In this case, if the coaxial converter 310 and an inner conductor 315 are integrally formed as one body, the inner conductor 315 integrally formed as one body with the coaxial converter 310 is displaced in a vertical upward direction of the processing container 100, following a displacement of the position of the rectangular waveguide 305 or the coaxial converter 310.
Meanwhile, the inner conductor 315 and the coaxial converter 310 allow a refrigerant to pass through the inner conductor 315 and are cooled even during the process. Thus, the temperature of the inner conductor 315 and the temperature of the coaxial converter 310 during the process are lower than the temperature of the outer conductor 340 and the temperature of the rectangular waveguide 305. Thus, the thermal expansion rate of the inner conductor 315 and the coaxial converter 310 during the process is lower than the thermal expansion rate of the outer conductor 340 and the rectangular waveguide 305.
As such, after a temperature increase, a taper connector 320 connected to the inner conductor 315 is displaced in an upper direction away from the RLSA 905 together with the inner conductor 315, and a gap between the taper connector 320 and the wavelength-shortening plate 905a, a gap between the wavelength-shortening plate 905a and the RLSA 905, and a gap between the wavelength-shortening plate 905a and the cooling jacket 210 vary. Thus, a transmission path of the microwaves varies, and a mode of the microwaves is unstable, and plasma becomes non-uniform. As such, the stability and reliability of the microwave plasma processing apparatus are poor.